It is a common mistake that people get mixed up between backup and backup.

back up – is used as a verb; is a phrasal verb

    The girls always back up all their work on the computer.
    I'll back you up if you need help just call me.
    The truck backed up very slowly.
    The sink backed up and water got all over the floor.

backup – is used as an adjective and noun

    The girls made a backup copy of their work.
    Did you remember to make a backup copy of the project?
    Do you have a backup copy for of the contract?
    I have a portable hard disk, and I back up all my work on my portable hard disk.

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